The present invention relates to a method for inspecting the quality of an optical disc on which data are recorded together with error detection and correction marks for detection errors and for correction thereof.
The quality of an optical digital disc such as a compact disc has been checked by detecting a block error rate (BLER) dependent on the error detection marks, that is the occurence rate of errors in a block of a disc within a set time.
Referring to FIG. 4, an optical pickup 2 for reading data and detecting the BLER of an optical disc 1 has a semiconductor laser diode 3 which emits a laser beam L perpendicular to the surface of the disc 1. The laser beam L is focused on the recording surface of the disc 1 through a beam splitter 4 and an objective 5. The beam reflected on the optical disc 1 passes through the objective 5 and the beam splitter 4 and is received by a photodetector 6.
Errors in each block of the disc included in the reproduced data from the photodetector 6 are counted. When the added error count per time unit is under a predetermined threshold value, the disc is determined as an appropriate product. On the other hand, when the added error count is larger than the threshold value, the disc is determined as an inferior product.
In addition, the disc may further have a warp as shown in FIG. 5. When there is such a warp, the BLER increases.
FIG. 6 is a graph showing a relationship between the BLER and a skew angle .theta. (FIG. 5) which is an angle between the axis of the laser beam L and a perpendicular of the surface of the disc. In the case of a disc a, if the skew angle increases, the BLER does not rapidly rise. On the other hand, the BLER of a disc b largely increases with the increase of the skew angle. However disc b passes the conventional inspection as long as the BLER remains under a threshold value TV at a certain detecting point of the disc.
A high density disc has a small track pitch, and hence small pits. Such a high density disc must be reproduced using a laser beam which forms a small beam spot on the surface thereof. Namely, the wavelength of the laser beam must be short and the aperture number of the objective must be increased. As a result, the depth of focus is decreased. Thus, it is impossible to accurately read from and write on a disc such as the disc b, where the width between the line showing the characteristics of the BLER is small. Namely, a range of the permissible skew angles, hereinafter referred to as a skew margin, is too small for the high density disc. Therefore, it is necessary to eliminate such a disc at the inspection.